The coding or decoding of video data with associated audio and/or other data can be performed, e.g., on the basis of the standards ISO/IEC 11172 (MPEG1) or ISO/IEC 13818 (MPEG2). In some cases, it is desirable to convert a bit stream coded on the basis of such standards into a bit stream with a different data rate. It is known from the standard ISO/IEC 13818-2, subclause 7.8, "SNR scalability", to generate for this purpose a data stream with two layers, i.e., a corresponding decoder processing either only one layer with low resolution or both layers with an increased resolution. In such a case, the second layer additionally contains DCT coefficients to represent the increased spatial resolution. However, the coding in such a case must already generate a spatial layer and there is obtained a bit stream to be decoded which has only two possible, fixed data rates or resolutions.
To permit other data rates as well, during the transcoding of the digital video bit stream, a processing of the primary bit stream should take place in such a way that a lower data rate of the secondary bit stream is possible along with simultaneous high picture quality. The best picture quality is achieved by a decoding of the primary bit stream and subsequent encoding into a secondary bit stream. However, extensive hardware is required for this purpose.
Accordingly, it is an object of an aspect of the present invention to specify a method for transcoding bit streams with video information including motion information, in which the output data rate or resolution can be varied within broad limits and in which only a minimal amount of hardware is required.
Data-reducing coding methods for video signals often use motion prediction in conjunction with a corresponding motion interpolation. For this purpose, motion vectors are determined for picture-element blocks or macroblocks respectively comprising a plurality of picture-element blocks. Depending on the type of these blocks or video data, e.g., field, I, P or B, numerous prediction modes are possible. For achieving an optimum coding, all these modes are taken into consideration.
The method and apparatus according to the present invention is based upon the reuse of motion-compensating prediction vectors of the primary bit stream in the secondary bit stream. Since the information content of the motion vectors from the primary bit stream retains its validity in the secondary bit stream, it is not necessary to recalculate motion vectors for transcoding. Instead, an encoding of the secondary bit stream is performed with the aid of the motion vectors from the primary bit stream. In this case, the encoding of the secondary bit stream is carried out in a restricted prediction mode, i.e. in the secondary bit stream only prediction modes which are described by the respective block type or the motion information of the primary bit stream, are authorized. The postprocessing of the prediction error, e.g., quantization, which on account of the altered data rate is likewise altered, is adjusted adaptively to the desired target data rate. Consequently, a lower data rate can also actually be ensured. The delay of the overall signal by the required processing time for the bit stream transcoding is also advantageously reduced by the present invention in comparison with a complete decoding/encoding.
The video signals may comprise luminance and/or chrominance signals. In principle, the method according to the invention for transcoding a bit stream with video data, to which motion information and video data type information are assigned, from a first data rate into a second data rate comprises of the incoming video data corresponding to the first data rate, are at least partially decoded with the aid of the motion information and video data type information obtained therefrom, and are encoded using the same motion information and video data type information into the video data corresponding to the second data rate, i.e., a motion prediction and interpolation in which the number of possible prediction modes is restricted to the prediction modes allowed for the respective video data type being used for the encoding.
In principle, the device according to the invention for transcoding a bit stream with video data, to which motion information and video data type information are assigned, from a first data rate into a second data rate, is provided with:
buffer-storing means and analyzing means, which obtain the motion and video data type information from the video data;
followed by video data decoding means, assigned to the first data rate, which include motion prediction and motion interpolation means controlled by the motion and video data type information;
followed by video data encoding means assigned to the second data rate which include motion prediction and motion interpolation means controlled by the same motion and video data type information, and in which the number of possible prediction modes is restricted to the prediction modes allowed for the respective video data type.